Weight paint, I can't subtract but can draw
With weight painting, you aren't actually painting any colour, you are just using painting tools to get a less technical approach to assigning values to each vertex. The colours you see are a visual indication of the value assigned, blender uses a colour ramp to indicate the value assigned, which is a range from 0.0 to 1.0.



The weight setting is the weight value you are assigning to each vertex, the strength will determine how many brush strokes it takes to get there. To remove the weight from a vertex, set it's weight to 0.0.